The Lowell Wastewater Treatment Facility released untreated or partially treated wastewater into the Merrimack River between 3:20 and 6:18 p.m. on Monday.
The Andover Health Division is warning residents to avoid contact with the river for 48 hours. The sewerage discharge increases health risks from bacteria and other pollutants.
Overflows occur when stormwater runoff is too much for the sewage system to process. As of 2:10 p.m. Monday, Andover had received 0.78 inches of rainfall in the previous 24 hours, according to the National Weather Service in Boston.